1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which can realize an image display of high quality by suppressing a defective display derived from the misalignment of a pair of substrates.
2. Description of the Related Art
A liquid crystal display device has been popularly used as a thin and light-weighted display device having high definition and capable of performing a color display for a note-book type computer, a display monitor of a television receiver set. Liquid crystal display panels constituting the liquid crystal display devices of this type are roughly classified into those of a single matrix type interposing a liquid crystal layer between a pair of substrates both of which form parallel electrodes arranged to intersect each other on respective inner surfaces thereof and those of an active matrix type having switching elements for selecting respective pixel units formed on one of a pair of substrates.
As the active matrix type liquid crystal display panel, a so-called vertical electric field type (generally referred to as “TN method”) which forms groups of electrodes for pixel selection on a pair of upper and lower substrates respectively as represented by a twisted nematic (TN type) and a so-called a lateral electric field type (generally referred to as “IPS type”) which forms a group of electrodes for pixel selection on only one of a pair of upper and lower substrates are known.
In the former TN type liquid crystal display panel, liquid crystal is oriented with a twisting of 90°, for example, within a pair of substrates (first substrate (lower substrate) and second substrate (upper substrate)), and two polarizing plates are laminated to outer surfaces of the upper and lower substrates of the liquid crystal display panel such that their absorption axis directions are arranged in a cross Nicol, and the incident-side absorption axis is arranged parallel to or perpendicular to the rubbing direction.
In such a TN type active matrix liquid crystal display panel, an incident light assumes a linear polarized light at the incident-side polarizing plate when a voltage is not applied, and the linear polarized light is propagated along the twisting of the liquid crystal layer. When a transmission axis of the irradiation-side polarizing plate is aligned with an azimuth angle of the whole linear polarized light, the linear polarized light is wholly irradiated thus providing a white display (so-called normally open mode). On the other hand, when the voltage is applied, the direction of a unit vector (director) which indicates the average orientation direction of axes of liquid crystal molecules which constitute the liquid crystal layer is directed in the direction perpendicular to a surface of the substrate and is aligned with an absorption axis of the irradiation-side polarizing plate since the azimuth angle of the incident-side linear polarized light is not changed thus providing a black display (see “Ekisho no Kiso to Oyo (Basic and Application of Liquid Crystal)” published by Kogyo Chosa Kai in 1991).
On the other hand, in the IPS type liquid crystal display panel in which a group of electrodes and a group of electric wiring for selecting pixels are formed on only one of a pair of substrates and the switching of the liquid crystal layer is performed in the direction parallel to the surface of the substrate by applying a voltage between neighboring electrodes (between the pixel electrode and the counter electrode) on the substrate, polarizing plates are arranged such that a black display is provided when the voltage is not applied (so-called normally closed mode).
In the liquid crystal layer of the IPS type liquid crystal display panel, the director of the liquid crystal layer which is arranged in a homogeneous orientation parallel to the surface of the substrate in the initial state and is arranged on a plane parallel to the substrate is arranged parallel to the direction of electrode wiring or makes a certain angle with respect to the direction of electrode wiring when the voltage is not applied, the direction of the director of the liquid crystal layer is shifted in the direction perpendicular to the electrode wiring direction along with the applying of the voltage when the voltage is applied. When the direction of the director of the liquid crystal layer is inclined in the electrode wiring direction by 45° with respect to the direction of the director when the voltage is not applied, the liquid crystal layer at the time of applying the voltage has an azimuth angle of the polarized light rotated by 90° as if a ½ wavelength plate so that a transmission axis of the irradiation-side polarizing plate and the azimuth angle of polarization are aligned thus providing the white display. This IPS type liquid crystal display panel is characterized in that the change of hue and contrast is small also in a viewing angle so that it is possible to obtain a wide viewing angle (see Japanese Laid-open Patent Publication 505247/1993).
With respect to the active matrix type liquid crystal display device among the above-mentioned liquid crystal display devices using various types of liquid crystal display panels, a black matrix (BM) which partitions respective pixels is formed on one of a pair of substrates so as to enhance the contrast. Particularly with respect to the TN type liquid crystal display device, switching elements such as thin film transistors (TFT) or the like are formed on a lower substrate side which constitutes the first substrate, color filters are formed on an upper substrate side which constitutes the second substrate, and the black matrix BM is provided around respective color filters formed on the second substrate. Further, a light shielding film is formed such that the light shielding film covers signal lines which supply driving signals to the switching elements formed on the first substrate so as to enhance the contrast of display images.
FIG. 19A, and FIG. 19B are schematic views showing a pixel portion of the TN type liquid crystal display device in an enlarged form, wherein FIG. 19A is a plan view and FIG. 19B is a cross-sectional view taken along a line A—A of FIG. 19A. With respect to this liquid crystal display device, on a main surface of a first substrate (lower substrate) SUB1, thin film transistors, pixel electrodes, video signal lines and scanning signal lines are formed, while on a main surface of a second substrate (upper substrate) SUB2, a black matrix, color filters disposed in regions surrounded by the black matrix and common electrodes (none of them shown in FIG. 19A and FIG. 19B) are arranged. In FIG. 19A and FIG. 19B, Xn, Xn+1 indicate video signal lines, Yn, Yn+1 indicate scanning signal lines, PIX indicates pixel electrodes, TFT indicates thin film transistors which constitute switching elements, and DCN indicates disclination areas (contrast reduced areas due to leaking of light) which are generated around the pixels. Orientation films which are brought into contact with a liquid crystal layer LC are respectively formed on uppermost layers of respective main surfaces of a pair of substrates. However, these orientation films are omitted in FIG. 19A and FIG. 19B.